Arcuate Seed Casting Method

ABSTRACT

A casting method includes: forming a seed, the seed having a first end and a second end, the forming including bending a seed precursor; placing the seed second end in contact or spaced facing relation with a chill plate; contacting the first end with molten material; and cooling and solidifying the molten material so that a crystalline structure of the seed propagates into the solidifying material. The forming further included reducing a thickness of the seed proximate the first end relative to a thickness of the seed proximate the second end.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 17/062,035,filed Oct. 2, 2020, and entitled “Arcuate Seed Casting Method” whichclaims benefit of U.S. Patent Application No. 62/911,121, filed Oct. 4,2019, and entitled “Arcuate Seed Casting Method” and U.S. PatentApplication No. 62/936,196, filed Nov. 15, 2019, and entitled “ArcuateSeed Casting Method”, the disclosures of which are incorporated byreference herein in their entireties as if set forth at length.

BACKGROUND

The disclosure relates to gas turbine engines. More particularly, thedisclosure relates to manufacture of disks and other annular components.

In typical manufacture of nickel-based superalloy disks (e.g., for gasturbine engine turbine sections or high pressure compressor (HPC)sections), manufacture is by forging of powder metallurgical (PM) orcast forms.

In distinction, only casting techniques are typically used to formblades, vanes, and combustor panels. Many blades are manufactured bysingle crystal casting techniques. In an exemplary single crystalcasting technique, a seed of single crystal material is used to define acrystalline orientation that propagates into the cast blade alloy as itcools and solidifies.

In casting blades, etc., it is well known that removal of high anglegrain boundaries (>10°) in single crystal nickel base superalloys leadsto improved creep resistance and consequently enhances its temperaturecapability. In addition, it is also known that by properly orienting thelow modulus <100> direction along the direction in which high thermalstrain exists, the thermal mechanical fatigue (TMF) capability of thematerial can also be significantly improved.

However, direct application of nickel base superalloy single crystal toa component such as a turbomachine disk, shaft, spacer, or the like, hasnot been practical. This is so because loading of such components due tohigh rotation speed around an axis is axially symmetric and will lead touneven strain distribution in a single crystal body, with cubic symmetryand anisotropic elastic and plastic properties.

U.S. Pat. No. 10,369,625, of Shah et al., Aug. 6, 2019, and entitled“Arcuate directionally solidified components and manufacture methods”,(the '625 patent, the disclosure of which is incorporated by referenceherein in its entirety as if set forth at length) discloses use of anarcuate seed to cast arcuate components. For example, an annular seedmay be used to cast an annular component such as a disk shaft, spacer,or the like.

SUMMARY

One aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end, theforming including bending a seed precursor; placing the seed second endin contact or spaced facing relation with a chill plate; contacting thefirst end with molten material; and cooling and solidifying the moltenmaterial so that a crystalline structure of the seed propagates into thesolidifying material. The forming further comprises reducing a thicknessof the seed proximate the first end relative to a thickness of the seedproximate the second end.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the seed and the moltenmaterial being nickel-based superalloys.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the solidus of the seed beingno more than 25° C. higher, if at all, than the solidus of an initialpour portion of the molten material.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the seed and the moltenmaterial being nickel-based superalloys.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the reducing comprisingelectrical discharge machining (EDM).

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the reducing being before thebending.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the reducing being after thebending.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the reducing being along aninner diameter face and an outer diameter face of the seed.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the reducing being at aplurality of locations circumferentially spaced in the ultimate bentseed.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the reducing creating athrough cut at each of the locations, optionally open to the first endof the seed.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the molten materialsolidifying in an annular form.

Another aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end, theforming including bending a seed precursor; placing the seed second endin contact or spaced facing relation with a chill plate; contacting thefirst end with molten material; and cooling and solidifying the moltenmaterial so that a crystalline structure of the seed propagates into thesolidifying material. The forming further comprises reducing a thicknessof the bent seed precursor by removing at least 20% of the precursorthickness from both an inner diameter surface and an outer diametersurface.

Another aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end, theforming including bending a seed precursor; placing the seed second endin contact or spaced facing relation with a chill plate; contacting thefirst end with molten material; and cooling and solidifying the moltenmaterial so that a crystalline structure of the seed propagates into thesolidifying material. The forming further comprises reducing a thicknessof the seed precursor at a plurality of spaced locations.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the reduced thicknessfacilitating the bending.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include at least one of: the reducedthickness being along a full height of the seed between the first endand the second end; the reducing of the thickness being at nineteen ormore locations; and the bending at least partially closing grooves atthe locations.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include at least one of: the reducedthickness being along only a partial height of the seed between thefirst end and the second end; the reduced thickness relieving stress inthe seed precursor proximate the first end; the reducing of thethickness being at nineteen or more locations; and the reducing of thethickness being after the bending.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the spaced locations includingfirst locations on a first face of the seed precursor and secondlocations on a second face of the seed precursor, the first locationscircumferentially offset from the second locations in the seed.

Another aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end, theforming including bending a seed precursor; placing the seed second endin contact or spaced facing relation with a chill plate; contacting thefirst end with molten material; and cooling and solidifying the moltenmaterial so that a crystalline structure of the seed propagates into thesolidifying material. The forming further comprises at least one of:rendering a portion of the seed precursor proximate the first end lessprone to stress from the bending than a portion of the seed precursorproximate the second end; relieving a stress in the seed proximate thefirst end relative to a stress in the seed proximate the second end; andremoving stressed material in the seed proximate the first end relativeto stressed material in the seed proximate the second end.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include: the forming including saidrendering; and said rendering comprising removing material.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the forming including saidrelieving and said relieving comprising removing material.

Another aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end, theforming including bending a seed precursor; placing the seed second endin contact or spaced facing relation a chill plate; contacting the firstend with molten material; and cooling and solidifying the moltenmaterial so that a crystalline structure of the seed propagates into thesolidifying material. The forming further comprises inserting the bentseed precursor into a sleeve leaving the bent seed precursor protrudingfrom a first end of the sleeve.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the sleeve holding the bentseed precursor compressed.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the sleeve having a secondend, the bent seed precursor has a first end and a second end, theprotruding being of a portion of the bent seed precursor proximate thefirst end of the bent seed precursor, the second end of the bent seedprecursor captured by an internal shoulder of the sleeve.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the sleeve having a secondend, the bent seed precursor has a first end and a second end, theprotruding being of a portion of the bent seed precursor proximate thefirst end of the bent seed precursor, the second end of the bent seedprecursor captured by a slot of the sleeve.

Another aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end and aninner diameter (ID) surface and an outer diameter (OD) surface; placingthe seed second end in contact or spaced facing relation with a chillplate; contacting the first end with molten material; and cooling andsolidifying the molten material so that a crystalline structure of theseed propagates into the solidifying material. The seed comprises atleast one first piece and at least one second piece. The formingcomprises inserting the at least one first piece into the at least onesecond piece, leaving the at least one first piece protruding from afirst end of the at least one second piece. The contacting the first endwith molten material contacts the molten material with the protruding atleast one first piece.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the at least one first piecebeing a single full annulus piece.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the at least one first piecehaving multiple layers in a radial direction.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the multiple layers beingformed by a spiral.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the multiple layers beingformed by radially stacking separate pieces.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the at least one first piecebeing a single piece, the at least one first piece being bent into abent condition, and the inserting being in the bent condition.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the at least one first piecebeing a plurality of pieces and the inserting comprising inserting eachof the plurality of pieces into a slot of the at least one second piece.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the at least one first piecebeing an SX nickel-based superalloy and the at least one second piecenot being an SX nickel-based superalloy.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the at least one second piecebeing an equiax nickel-based superalloy.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the at least one first piecebeing a Ni-based alloy and the at least one second piece being a steel.

Another aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end and aninner diameter (ID) surface and an outer diameter (OD) surface;contacting the first end with molten material; and cooling andsolidifying the molten material so that a crystalline structure of theseed propagates into the solidifying material. The forming comprisesforming the seed with multiple layers in a radial direction.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the multiple layers beingformed by a spiral.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the multiple layers beingformed by radially stacking separate pieces.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the multiple layers being atleast three layers at some given position.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the multiple layers forming aseed upper portion and the method comprising inserting the seed upperportion into a seed lower portion.

Another aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end and aninner diameter (ID) surface and an outer diameter (OD) surface; placingthe seed second end in contact or spaced facing relation with a chillplate; contacting the first end with molten material; and cooling andsolidifying the molten material so that a crystalline structure of theseed propagates into the solidifying material. At least a portion of theseed contacted with the molten material has a solidus higher than asolidus of at least an initial pour portion of the molten material.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the solidus of the initialpour portion being lower than a solution temperature of said portion ofthe seed.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the solidus of the initialpour portion being at least 25° C. lower than the solution temperatureof said portion of the seed.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the solidus of said portion ofthe seed being at least 25° C. higher than the solidus of the initialpour portion.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the solidus of said portion ofthe seed is at least 25° C. higher than the solidus of the initial pourportion.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the solidus of said portion ofthe seed being 25° C. to 200° C. higher than the solidus of the initialpour portion.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the seed and the moltenmaterial being nickel-based superalloys.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include a subsequent pour portion ofthe molten material having a solidus higher than the solidus of theinitial pour portion. After the solidifying, the initial pour portionand remainder of the seed are removed.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the solidus of the subsequentpour portion of the molten material being 25° C. to 200° C. higher thanthe solidus of the initial pour portion.

Another aspect of the disclosure involves a casting method comprising:forming a seed, the seed having a first end and a second end and aninner diameter (ID) surface and an outer diameter (OD) surface; placingthe seed second end in contact or spaced facing relation with a chillplate; contacting the first end with molten material; and cooling andsolidifying the molten material so that a crystalline structure of theseed propagates into the solidifying material. The forming of the seedincludes applying a melting point depressant to a seed precursor alongat least a portion of the seed contacted with the molten material.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the melting point depressantcomprising boron. The melting point depressant produces a remelttemperature lower than a recrystallization temperature.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the melting point depressantbeing applied by vapor deposition.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the melting point depressantbeing applied by slurry coating.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the melting point depressantcomprising a powder mixture of alloys of at least two differentcompositions.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the melting point depressantbeing applied on a compressively-strained inner diameter surface and atensile strained outer diameter surface.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the melting point depressantbeing applied on a top surface.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the melting point depressantbeing effective to increase a height of melt back along at least one ofan inner diameter surface and an outer diameter surface.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the seed precursor having asolidus no more than 25° C. higher, if at all, than a solidus of themolten material

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the seed precursor and themolten material being essentially identical compositions.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the seed precursor and themolten material being nickel-based superalloys.

A further embodiment of any of the foregoing embodiments or aspects mayadditionally and/or alternatively include the molten materialsolidifying as a full annulus component.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial central vertical/axial sectional view of a castingmold (shell) with a seed prior to an alloy pour.

FIG. 2 is a view of the seed.

FIG. 3 is a view of the mold after the alloy pour.

FIG. 4 is a view of an alternative seed

FIG. 5 is a top view of an alternative seed.

FIG. 6 is a top view of an alternative seed.

FIG. 7 is a view of an alternative seed.

FIG. 8 is a partial central vertical/axial sectional view of the seed ofFIG. 7.

FIG. 9 is a partial central vertical/axial sectional view of a firstvariation on the seed of FIG. 7.

FIG. 10 is a partial central vertical/axial sectional view of a secondvariation on the seed of FIG. 7.

FIG. 11 is a partial top view of the seed of FIG. 10.

FIG. 12 is a partial central vertical/axial sectional view of a thirdvariation on the seed of FIG. 7.

FIG. 13 is a partial top view of the seed of FIG. 12.

FIG. 14 is a view of a further seed.

FIG. 15 is a partial central vertical/axial sectional view of the seedof FIG. 14.

FIG. 16 is a partial central vertical/axial sectional view of a firstvariation on the seed of FIG. 14.

FIG. 17 is a partial central vertical/axial sectional view of a castingmold (shell) with a surface-treated seed prior to an alloy pour.

FIG. 18 is a view of the FIG. 17 mold after the alloy pour.

FIG. 19 is a partial central vertical/axial sectional view of a castingmold (shell) with a surface-treated seed prior to an alloy pour.

FIG. 20 is a view of the FIG. 19 mold after the alloy pour.

FIG. 21 is a partial micrograph of a baseline casting including seedremnant.

FIG. 22 is a partial micrograph of a second casting including seedremnant.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

When using a bent arcuate seed of a given alloy to cast that same alloy,recrystallization of the bent seed interferes with the successfulcasting of single-crystal (SX) and directionally solidified (DS)castings. The bending of the seed introduces stresses and strains acrossthe seed thickness. Along an inner diameter (ID) depthwise region, thestresses and strains are principally compressive. Along an outerdiameter (OD) depthwise region, the stresses and strains are principallytensile. The strains may include a substantial plastic component.

In casting, the bent seed has a tendency to recrystallize due to theplastic deformation. The recrystallized microstructure of the seed, inturn, propagates into the solidifying pour instead of the desiredsingle-crystal structure.

Tables I and II below respectively identify temperature properties andnominal composition for exemplary nickel-based superalloys discussedbelow as seeds and/or castings. Such alloys typically have at least 45%Ni by weight (e.g., 48% to 65% or 50% to 65%). Typically, elements otherthan those listed in Table I will be up to 10.0 wt. % aggregate (morenarrowly 5.0 wt. %) and up to 1.0% individually (more narrowly 0.5 wt.%). Minor elements in the table may, in various implementations, bepresent at higher levels such as for such other elements. Nevertheless,other alloys may be used.

TABLE I Exemplary Alloy Properties Solution Solidus Liquidus temperatureAlloy (° F. (° C.)) (° F. (° C.)) (° F. (° C.)) IN 718 2319-2224* 2437(1336) 1750 (954)  (1271-1218) Alloy A 2446 (1341) 2511 (1377) 2380(1304) Alloy B 2382 (1306) 2485 (1363) 2387 (1308) Alloy C 2271 (1244)2543 (1395) 2286 (1252) Alloy D 2197 (1203) 2481 (1361) 2220 (1216)Alloy E 2188 (1198) 2418 (1326) 2050 (1121) Alloy F 2294 (1256) 2511(1377) 2200 (1204) Udimet ™ 720LI 2105 (1151) 2453 (1345) 2075 (1135)Alloy G 2207 (1208) 2374 (1301) 2165 (1185) *Depending on particularcommercial variety—highly dependent on boron content.

From Table I, it is seen that several of the example alloys havesolution temperature higher than solidus while others have solutiontemperature lower than solidus.

Table II Exemplary Alloy Nominal Composition (weight percent) Alloy C SiMn P S Al B Co Cr Cu. Fe Hf Mo IN 718 0.05 0.13 0.07 <0.005 0.001 0.550.005 0.00 19.00 0.00 18.50 0.00 3.00 Alloy A 0.001 <0.12 <0.12 <0.015<0.015 5.65 0.000 10.00 5.00 0.05 0.25 0.10 1.90 Alloy B 0.001 <0.12<0.12 <0.015 <0.015 5.00 0.003 5.00 10.00 0.05 0.25 0.00 0.00 Alloy C0.10 0.10 0.10 0.005 0.002 6.00 0.015 12.00 6.50 0.05 0.10 1.50 2.00Alloy D 0.14 0.10 0.10 0.005 0.002 5.00 0.002 10.00 9.00 0.05 0.25 2.000.00 Alloy E 0.1 <0.12 <0.12 <0.015 <0.015 3.6 0.013 9.0 12.0 <0.1 <0.2<0.1 1.9 Alloy F 0.15 0.10 0.10 0.005 0.002 5.50 0.015 10.00 8.40 0.050.25 1.40 0.70 UDIMET 720Li 0.014 0.00 0.00 0.00 0.00 2.51 <0.03 14.616.2 0.00 <0.2 0.00 2.9 Alloy G 0.04 <0.06 <0.02 <0.006 0.006 3.41 0.02720.9 10.45 <0.010 0.25 <0.05 3.3 Alloy Nb Ni Re Ta Ti V W Zr IN 718 5.0052.79 0.00 0.00 0.90 0.00 0.00 0.00 Alloy A 0.00 59.75 3.00 8.40 0.000.00 5.90 0.00 Alloy B 0.00 62.19 0.00 12.00 1.50 0.00 4.00 0.00 Alloy C0.00 58.50 3.00 4.00 0.00 0.00 6.00 0.03 Alloy D 1.00 58.35 0.00 0.002.00 0.00 12.00 0.00 Alloy E 0.00 Bal. 0.00 5.0 4.1 0.00 3.8 <0.02 AlloyF 0.00 59.07 0.00 3.10 1.10 0.00 10.00 0.06 UDIMET 720Li 0.00 Bal. 0.000.00 4.97 0.00 1.2 <0.06 Alloy G 1.85 Bal. 0.00 4.40 3.00 <0.1 2.00 0.00

Because recrystallization is a function of strain, the recrystallizationtemperature will vary between specific applications as a function of thestrain state (gradient). The specific recrystallization temperature canbe determined by exposing sections of the seed plate to highertemperatures in increments (e.g., 25° F. (14° C.)) untilrecrystallization is observed metallographically. Further testing withsmaller increments may refine the result. Table I identifies a solutiontemperature which may be used to predict recrystallization properties(discussed below).

FIG. 21 shows an SX seed 800 of a first alloy after casting a body 802of a second alloy. In this photo, both alloys are Alloy A. Prior tocasting, the seed 800 extended from a lower end 804 to an upper endshown by dashed line 806. The seed had been bent to define an innerdiameter (ID) surface 808 and an outer diameter (OD) surface 810extending between the lower end and upper end.

A melt back region 820 extends between respective lower and upper limits822 and 824. Below the melt back region, along both the aforementionedID depthwise region and OD depthwise region are recrystallization zones830 and 832 leaving a relatively unaffected region 836 radially inbetween. The two recrystallization zones 830 and 832 thus propagate therecrystallized structure up into the body 802 causing associatedportions of the body 802 to lack the desired relatively puremicrostructure of the original unaffected seed 800.

Several mechanisms are theorized for mitigating or avoiding this effect.A number of them involve modifying seed geometry/construction. Othersinvolve seed material properties (meting/solidifying andrecrystallization temperatures).

A first mechanism involves removing the strained/stressed regions afterbending. This may involve removal of an entire layer (circumferentialextent) in a vertical region expected to reach recrystallizationtemperature. Because the lower end of the seed contacts (or is in closefacing relation to) the chill plate, only an upper portion should reachany relevant temperature. Thus, removal may be of an entire ID layer andan entire OD layer along an upper portion while leaving those layersalong the lower portion.

FIG. 1 shows such a modified SX seed 20 extending from a lower end 22 toan upper end 24. The lower end rests atop an upper surface 103 of achill plate 102 of a furnace (or is in close facing spaced relation).The seed 20 is laterally embedded in a ceramic shell 104 having an uppermold cavity 106 above the seed. A gap (not shown) may be engineeredbetween the seed and chill plate to control heat transfer in at leastsome conditions (e.g., depending on the nature of the casting and theequipment). The exemplary seed 20 and shell 104 are generally (e.g.,subject to segmentation issues discussed below) bodies of revolutionabout a vertical axis 10 (spacing not to scale). The exemplary shell 104is a two-piece shell (with further pieces including pour cones, etc., ifany, not shown) having an inner piece 104A concentrically surrounded byan outer piece 104B.

As discussed above, the seed has a lower portion or section 30 and anupper portion or section 32. The seed has an inner diameter (ID) surfaceand an outer diameter (OD) surface. Along the lower portion, the IDsurface 34 is an intact portion of a surface of an original SX stripbent to form the seed. Similarly, along the lower portion, the ODsurface 36 is an intact portion of the opposite face of the original SXstrip. The original strip (seed precursor) thickness was T₁. Along theupper portion 32, the ID surface 34′ and OD surface 36′ have beenrelieved relative to the corresponding surfaces 34 and 36. Exemplaryrelief is shown by respective thicknesses T₂ and T₃ leaving a thicknessT_(U) remaining.

In exemplary embodiments where both T₂ and T₃ are nonzero, each are anexemplary at least 5% (more narrowly at least 10% or at least 20% or 5%to 35% or 10% to 30% or 20% to 50% (thus allowing for ID v. OD asymmetryof removal)) of the original thickness T₁. In other embodiments (notshown), only one of T₂ and T₃ is nonzero.

In absolute terms for typical part sizes in the range of 0.1 m to 2.0 min diameter, exemplary T₁ is 2.0 mm to 20.0 mm (more particularly 2.0 mmto 10.0 mm) and exemplary T₂ and T₃ are at least 0.5 mm (moreparticularly 0.5 mm to 8.0 mm or 0.5 mm to 4.0 mm or 1.0 mm to 3.0 mm).Maximum diameter may be influenced by furnace availability with typicalcommercial furnaces able to go up to about 1.0 m.

Exemplary thinning is a machining such as electrical discharge machining(EDM), abrasive grinding, or high speed milling. Although the seedcavity of the FIG. 1 shell is stepped to closely accommodate the steppedseed, other configurations are possible. Due to the vertical thermalgradient, the warmer upper portion of the seed will tend todifferentially thermally expand relative to the lower portion. Viewed incross section, and relative to temperature before heating of the seed,due to the heating the upper portion of the seed will progressivelyflare radially outward from its junction with the lower portion to itsupper end. The shell may be dimensioned to accommodate this flaring.

FIG. 2 shows the seed 20 formed as a segmented seed (circumferentiallysegmented) rather than a single-piece seed. Exemplary segmentation is intwo 180° segments 40, each being a single bent SX piece. Exemplarysegments 40 each have a first circumferential end 42 and a secondcircumferential end 44 with the two segments positioned end-to-end withadjacent ends contacting (or bonded to each other or closelyspaced-apart). FIG. 2 also has a broken line showing of the originalstrip thickness along the upper portion 32. To hold the seed elasticallystrained, the pieces may be secured to each other along the lowerportion 30 (e.g., a weld, braze, or diffusion bond at the mating ends 42and 44 or fasteners).

Such a thinning to form the seed upper portion 32 may have an additionalbenefit of reducing stresses that the seed upper portion 32 applies tothe shell 104 due to differential thermal expansion.

FIG. 3 shows the shell 104 after casting with the body 50 of a secondalloy cast in the mold cavity and showing the approximate extent of amelt back region 52. Because the melt back region is limited to theupper portion 32 there is not the same propagation of recrystallizedstructure as with the seed 800. The seed alloy and second alloy may bethe same or different. For example, IN 718 cast using an Alloy A seed.

Variations of the above methods providing similar structure (and thusnot shown separately), are to eliminate the high stress/strain regionsbefore bending. This may involve a thinning of a portion of the metallicstrip to form what will become the seed upper portion 32. This leaves arelatively thick lower portion 30 for sufficient thermal mass andthermal contact with the chill plate. However, there are differentconsiderations when making such a seed by pre-bending material removal.For a given T₁, T₂, and T₃, material removal pre-bending does notachieve an identical result to removing material post-bending.

Several considerations indicate that the pre-bending removal situationmay leave higher magnitude stresses than the post-bending removal (ifany). First, contact with bending tooling may impart stresses incontacted areas. Material from these areas is removed in thepost-bending removal situation, but not the pre-bending removal. Second,even where T₂ and T₃ are effective to remove all stressed material (ormaterial above given stress thresholds) in a post-bending removal, in apre-bending removal T_(U) is still great enough that bending createsstressed depthwise zones exceeding the thresholds. Thus, a pre-bendingmaterial removal may be associated with a smaller T_(U) than is apost-bending material removal that produces a similar lack ofrecrystallized structure propagation. For this, exemplary T_(U) in apre-bending material removal is 20%-80% of the original thickness T₁,more particularly 20% to 50%.

This consideration of pre-bending removal v. post-bending removalhighlights the possibility of a full height post-bending removal by thethicknesses T₂ and T₃ providing benefit.

A second mechanism is to relieve stress by more selectively removingmaterial from the strained/stressed regions. In one example (FIG. 4) ofan SX seed 200 (one-piece or segmented—a two-segment example shown), acircumferential array of axial grooves or slits 202 may be machinedthrough one or both of the ID and OD strained/stressed regions. Thisallows relaxation of depthwise surface regions of intact material 204circumferentially between the grooves. As with the seed 20, the groovesmay only be partial height (limited to upper portion or section 210thereby providing maximum surface area of bottom of the lower portion orsection 212 in contact with the chill plate).

The FIG. 4 slits are through-slits (e.g., through-sawn or EDM).Alternatively to through-cutting, the grooving may leave a web ofmaterial (e.g., thickness similar to the portion 32 or smaller). FIG. 5is a partial top view of such a seed 230 otherwise similarly formed tothe seed 200 showing grooves 232 (shown both ID and OD, but may be ononly one of the two) leaving webs 234 between intact material.

Alternative embodiments with grooves both ID and OD could have themcircumferentially offset or staggered (e.g., FIG. 6 seed 235 where eachID groove is circumferentially intermediate the two adjacent OD groovesand vice versa). The grooves 232 may be full height (to ease bending ifformed prior to bending) or may be just partial height.

Exemplary partial depth grooving is a machining such as electricaldischarge machining (EDM), abrasive grinding, or high speed milling.

As noted relative to FIG. 6, other variations (not shown separately),are to similarly slit (FIG. 4) or groove (FIG. 5) before bending. Thisreduces the strain/stress on bending. The bending may fully or partiallyclose the original grooves or slits. It may be preferable to cut theslits 202 or grooves 232 before bending because more strain/stress willbe prevented by this than will be relieved by a post-bending slitting.Because the lower portion is below the ultimate melt back region, it isacceptable for the lower portion to have unrelieved depthwiseplastically stressed/strained regions along the ID surface 34 and ODsurface 36.

A third mechanism may alternatively involve using a thin sheet/strip andmating it to a thicker piece. The thin sheet/strip, once bent, forms theseed upper portion while the thicker piece (and lower portion of thestrip) form the seed lower portion. Held in this manner the thinnerpiece is elastically deformed, but in an exemplary embodiment is notplastically deformed. Lacking plastic deformation, the driving force forrecrystallization is removed. In one example, a seed 250 (FIG. 7) isformed from a full annulus thick lower piece 252 (or multiplecircumferential segments fastened end-to-end) and a single thin bentstrip upper piece 254. The lower piece 252 (FIG. 8) extends from a lowerend 260 to an upper end 262 and has an inner diameter (ID) surface 264and an outer diameter (OD) surface 266. Similarly, the upper piece 254extends from a lower end 270 to an upper end 272 and has an innerdiameter (ID) surface 274 and an outer diameter (OD) surface 276.

The upper piece 254 mounts within the lower piece 252 (e.g., so that thelower piece, via hoop stress, retains the upper piece in its circularfootprint). A lower portion 278 of the upper piece 254 is receivedwithin the lower piece and an upper portion 280 protrudes above thelower piece upper end 262. FIG. 7 shows a small gap 282 betweencircumferential ends 284 and 286 of the upper piece 254. In theexemplary embodiment of the seed 250, the lower piece 252 ID surface hasan upper rebate 268 (defining shoulder 269) to radially receive theupper piece 254 lower portion 278.

Alternative embodiments may replace the rebate/shoulder with a slot 294(FIG. 9 in lower piece 292 of seed 290). Slot radial span may beslightly greater than upper piece thickness to allow ease of insertionwhile holding the upper piece elastically strained/stressed.

The upper piece 254 may be relatively thin. Exemplary thickness is 0.008inch (0.20 mm), more broadly 0.003 inch to 0.10 inch (0.076 mm to 2.5mm) or 0.005 inch (0.13 mm) to 0.05 inch (1.3 mm).

The thin, elastically held, SX seed piece 254 may be a single SXpiece/layer. Alternatively, multiple layers may be built up in one ormore SX pieces radially stacked to align crystalline axes (FIGS. 10 and11 show seed 296 having multi-layer upper piece 297 as a variation onFIG. 8—a similar variation on FIG. 9 could be made). Exemplary alignmentof the layer orientations should be less than 8.0° or, preferably lessthan 6.0° or less than 4.0°. A one-piece, multiple-layer, construction(not shown) could be a spiral (FIGS. 12 and 13 show seed 400 havingspiral upper piece 402—e.g., cut from a larger stock SX coil to form aspiral having an inner end 404, an outer end 406, an inner diameter (ID)surface or face 408, and an outer diameter (OD) surface or face 410). Aconcentric radial stack could have each piece 298 (FIG. 11) forming onelayer with the gaps 299 of each piece/layer circumferentially offsetfrom gaps of the other piece(s)/layer(s) to provide good seeding alongthe full circumference. Alternatively, each layer may be multipleend-to-end pieces. A hybrid could involve a spiral formed fromend-to-end segments that might each be over 360° or well under 360°. Thespiral or concentric radial stack of pieces may define an exemplary twoto fifty layers (turns for a spiral), more narrowly two to ten or two tofive or three to five or at least two or at least three. This allowsbuildup to a greater total seed thickness than could otherwise beprovided maintaining elastic bending. Such a buildup may be up to nearthe full thickness of the lower piece 254.

Alternative built-up seeds (not shown) could avoid the lower piecealtogether (e.g., a spiral or stack alone held together by a weld orbraze or fastener).

Combined dimensions of the lower portion 278 and lower piece 254 maycorrespond to those of the lower portion 30 of the seed 20. However,manufacturing considerations may provide more flexibility in goingbeyond either end of that range.

The upper piece 254 and lower piece 252 may be the same material ordifferent. One example has the lower piece formed by machining such ashigh speed milling; whereas the upper piece is a bent SX strip. Thelower piece may differ from the upper piece in chemistry or crystallinestructure. For example, the lower piece may be a non-SX structure (e.g.,machined from billet or equiax cast to final or near final shape) of thesame material as the upper piece. It is seen that use of such differenttechniques allows the lower piece to be relatively thick without thesame extent of machining required for a single-piece embodiment.

The lower holding ring is most ideally a non-single crystal piece ofmaterial machined into shape. Many materials could serve this role withvarying advantages or disadvantages. More generally, a nickel-basednon-single crystal alloy lower piece such as IN 718 could be used for SXupper pieces formed of other nickel superalloys due to its reasonablecost and near match to the SX piece(s) in thermal expansion properties.Use of the same alloy for both sections would provide essentiallyperfect match for thermal properties but be more expensive than using amore common equiax nickel alloy such as IN718.

Alternatively, to minimize cost, a steel lower piece could be used(e.g., stainless such as 304 stainless). This would be low cost butwould have a thermal expansion mismatch with the SX nickel-basedsuperalloy upper piece(s). Alternatively, to maximize conductivity fromseed to chill plate, a copper (or other high thermal conductivity) lowerpiece may be used. This would have the disadvantage of thermal expansionmismatch relative to the SX piece, but would provide very high thermalconductivity from the chill plate to seed. Ultimately, choice ofmaterial for lower piece, including dimensions, will depend on detailsof the part being cast and equipment being used for casting.

A fourth mechanism may combine some form of segmentation of an uppermember with a lower member. In one example, a seed 300 (FIG. 14) isformed from a full annulus thick lower piece 302 and a circumferentialarray of SX upper pieces 304. The upper pieces combine to form a fullysegmented upper member 306. In the illustrated embodiment, adjacentpieces 304 may contact each other (e.g., may be cut as slight trapezoidsfor flat contact). In some embodiments (not shown), they are spacedapart by gaps. The associated shell may extend through the gaps toisolate the mold cavity from the lower piece 302.

The lower piece 302 extends from a lower end 320 to an upper end 322 andhas an inner diameter (ID) surface 324 and an outer diameter (OD)surface 326. Similarly, each upper piece 304 extends from a lower end330 to an upper end 332 and has an inner diameter (ID) surface 334 andan outer diameter (OD) surface 336. Each upper piece has a first lateralface 338 and a second lateral face 340.

The upper pieces 304 mount to the lower piece 302. A lower portion 342(FIG. 15) of each upper piece 304 is received within a receiving feature346 of the lower piece and an upper portion 344 protrudes above thelower piece upper end 322. In the exemplary embodiment, the receivingfeature of the lower piece is a circumferential slot as in the FIG. 10piece 292 separating ID and OD walls. An alternative feature (not shown)could be a shoulder. Or, the inner and outer walls could be separatepieces, leaving the lower ends to closely face or contact the chillplate upper surface.

A fifth mechanism involves relative solidus points and/orrecrystallization points. The alloy to be cast may have its solidus(temperature at which melting occurs) lower than the temperature atwhich seed material would recrystallize. The goal is to avoid the pourcausing an undesirable amount of seed recrystallization. The pourtemperature will be above the solidus of the pour alloy. If the poursolidus is too much higher (if higher at all) than the solutiontemperature of the seed, there will be recrystallization.Recrystallization occurs when the alloy exceeds solution temperature.For example, FIG. 22 shows the Alloy A seed 800 used to cast IN 718 body802′. Table I shows the IN 718 pour solidus as being slightly below theAlloy A seed solution temperature, thereby providing a sufficient marginto avoid undue recrystallization. In an ideal case one would expectsolution temperature to be lower than solidus temperature of an alloy.

Solution temperature is the temperature at which all the γ′ precipitatesare dissolved and the alloy becomes a single phase γ solid solution.Solidus temperature is the temperature at which the last fraction ofliquid solidifies. Owing to unavoidable dendritic segregation duringcasting, the solidus is generally associated with the interdendriticregion while the solution temperature is associated with the dendriticcore. So, depending on the alloy compositions, in some alloys solutiontemperature may be higher than the solidus temperature by few (typically<25° F. (<14° C.)) degrees. The difference also depends on theexperimental or theoretical model by which these numbers are determinedor estimated, respectively.

The required criteria of relative solidus of casting alloy andrecrystallization temperature of seed can be stated as the solidus ofthe cast alloy must be lower than the actual recrystallizationtemperature of the seed. While the recrystallization temperature can beestimated, the actual value for bent seed would be experimentallydetermined (e.g., testing on an actual bent seed or similar thickness ofsimilarly bent material) and is likely to be much higher. But, for easeof estimate, one or more proxy temperature differences may be used.

One proxy involves pour solidus and seed solution temperature discussedabove. Preferably the solidus should be lower than the solutiontemperature, more narrowly at least 45° F. (25° C.) lower. Upper limitson the range largely depend on the relative melting conditions allowingthe seed to serve as a seed. A likely typical upper limit would be 80°C. However, greater deltas such as 200° C. are possible.

Alternatively or additionally, a proxy solidus delta may be used.Exemplary solidus delta between IN718 and Alloy A is at least 70° C.,more particularly 70° C. to 125° C. A more generalized rangecomprehending other pairs may be at least 20° C. or at least 25° C. orat least 40° C., with any of those lower range ends associated withupper range ends, if any, of at most 100° C. or 150° C. or 200° C. Withthis in mind, it is seen that the prior mechanisms discussed above haveparticular use when this solidus/recrystallization or solidus/solidusrelationship is not present.

A sixth mechanism, also utilizing relative solidus points and/orrecrystallization points discussed above, is to use a sacrificialinitial pour 910 (FIG. 16) to intervene between the seed 800 and themain pour 920 of material of the ultimate cast part. The relationshipbetween the material properties of the sacrificial pour and both theseed and main pour may be similar to the examples above. The initialpour 910 (to a level or surface 912 and producing a first melt back zone914 below the original seed upper surface 806 at seed height H_(S)) isat a temperature below the seed 800 recrystallization temperature. Ingeneral, the alloy of the initial pour will have a lower melting point(solidus) than the seed alloy; whereas the main pour alloy will have ahigher melting point than the initial pour alloy. The seed and main pourmay be, but need not be, the same alloy.

The progressive withdrawal from the furnace means that the initial pourwill have partially solidified by the time the higher-temperature mainpour 920 begins (producing second melt back zone 924). The solidifiedportion is relatively unstrained, so recrystallization is not an issue.FIG. 16 shows a cut-off level 926 within the main pour. After casting,material below the level 926 (i.e., including seed remnant and theinitial pour material) is removed leaving the ultimate casting formedessentially (e.g., subject to minor diffusion, if any) by the main pour.

Although a single main pour 920 is shown, there may be multiple poursafter the sacrificial initial pour 910 and these multiple pours need nothave any special property relationship. Similar additional pours mayattend embodiments described both above and below.

A seventh mechanism involves use of a melting point suppressant(depressant) 950 (FIG. 17) on the seed. As discussed further below,variations may involve one or both of affecting melting of the seed andsolidification of the poured alloy. The FIG. 17 example is on both theID and OD surfaces of a seed (base seed or seed substrate which may bethe same as the seed 800 discussed above). Although initiallycounterintuitive relative to the examples above, the mechanism involveslowering melting temperature without similarly affectingrecrystallization temperature. By reducing the solidus belowrecrystallization temperature, the seed will melt back beyond therecrystallization regions (melt back region 952 in FIG. 18). Thus, thereis no recrystallization at the melt back front and the single crystalstructure will propagate into the solidifying pour.

An exemplary suppressant 950 is or contains boron (e.g., a boron-richnickel alloy). The suppressant may be applied to a seed precursor (e.g.,base seed 800 or precursor thereof) surface such as via vapordeposition, thermal spray, or slurry coating (e.g., spraying, brushing,or dipping). Thickness T_(C) of this layer will be a function ofsuppressant used and method of application. It may be atomic scaled forthe case of PVD application of boron (e.g., up to about 50 micrometers,more narrowly 5.0 micrometers to 50 micrometers) or macroscopic and asthick as 0.5-3.0 mm in the case of slurry coating methods (e.g., of apowder form braze material).

One group of powder form braze materials for the layer 950 is self-brazematerials. For self-brazing, it comprises a mixture of alloys of atleast two different compositions. At least one of the alloys is arelatively low melting point (low melt) alloy and at least one of thealloys is a relatively high melting point (high melt) alloy. Thesealloys may themselves be nickel-based superalloys. The low melt alloymay comprise a relatively higher content of one or more elements actingas melting point suppressants/depressants (e.g., boron) than does thehigh melt alloy.

U.S. Pat. No. 8,075,662 (the '662 patent) of Minor et al., issued Dec.13, 2011, the disclosure of which patent is incorporated by referenceherein in its entirety as if set forth at length, discloses exemplaryself-braze material formed of a mixture of powders of differingcomposition. Alternatively, just the high boron component of such amaterial could be used for the layer 950.

However, the effect on pour solidification may also be relevant. Meltingpoint depression of the pour may effectively cause a high melting pointpour alloy to behave as a low melting point alloy and not cause seedrecrystallization. FIG. 19 shows a seed having a melting pointsuppressant/depressant 970 (e.g., as discussed above and to a similarthickness) atop the base seed 800. For example, the depressant 970 maybe applied before or after insertion of the base seed 800 into theshell.

FIG. 20 shows a pour alloy whose normal solidus is higher than the baseseed solution temperature. Upon contacting the depressant the depressantabsorbs into the molten pour, reducing the pour solidus. The pour heatsthe remaining depressant and adjacent portion of the base seed. But thisinteraction cools the now solidus-depressed portion of the pour andmelted portion of the base seed to a temperature below the base seedsolution temperature, thereby avoiding recrystallization.

FIG. 20 shows a mixing zone 980 characterized by solidus depression(e.g., below the solidus of at least one of the base seed and pouralloy). The mixing zone upper end is shown at a level below the originaltop of the base seed. Above that is essentially pour alloy with a slightcompositional gradient.

Although examples are illustrated in the context of casting a fullannulus component, other arcuate components may similarly be cast frombent arcuate seed segments. For example, partial circumferencecylindrical or frustoconical segments may be cast for subsequentcircumferential assembly. Thus, the associated seeds may themselves beisolated arcuate segments rather than full or near essentially fullannulus (integral/continuous annulus or segmented). Exemplary segments,when used, may be bent by at least 5° and amounts all the way up to andbeyond 360° (e.g., 45° to 360° but many more times beyond 360° in thecase of spirals).

Further variations may reflect variations discussed in the '625 patent.Further variations may involve recombining features of the individualdescribed embodiments in any appropriate combination includingcombinations of the temperature delta manipulations and physical seedconfigurations and processing. For example, various of the physical seedconfigurations and processing steps may have particularly significantbenefit when the seed and pour material is relatively thermally matched(e.g. same material or material with close solidus-solidus orsolidus-solutioning relationships). Nevertheless, the physical seedconfigurations and processing steps may also be used when the seed andpour has the greater difference in thermal properties discussed for theother embodiments.

The use of “first”, “second”, and the like in the following claims isfor differentiation within the claim only and does not necessarilyindicate relative or absolute importance or temporal order. Similarly,the identification in a claim of one element as “first” (or the like)does not preclude such “first” element from identifying an element thatis referred to as “second” (or the like) in another claim or in thedescription.

Where a measure is given in English units followed by a parentheticalcontaining SI or other units, the parenthetical's units are a conversionand should not imply a degree of precision not found in the Englishunits.

One or more embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. For example, whenapplied to an existing baseline part configuration and/or castingprocess or apparatus, details of such baseline may influence details ofparticular implementations. Accordingly, other embodiments are withinthe scope of the following claims.

1. A casting method comprising: forming a seed, the seed having a firstend and a second end, the forming including bending a seed precursor;placing the seed second end in contact or spaced facing relation with achill plate; contacting the first end with molten material; and coolingand solidifying the molten material so that a crystalline structure ofthe seed propagates into the solidifying material, the forming furthercomprising: reducing a thickness of the seed proximate the first endrelative to a thickness of the seed proximate the second end.
 2. Themethod of claim 1 wherein: the seed and the molten material arenickel-based superalloys.
 3. The method of claim 1 wherein: the solidusof the seed is no more than 25° C. higher, if at all, than the solidusof an initial pour portion of the molten material.
 4. The method ofclaim 1 wherein: the seed and the molten material are nickel-basedsuperalloys.
 5. The method of claim 1 wherein: the reducing compriseselectrical discharge machining (EDM).
 6. The method of claim 1 wherein:the reducing is before the bending.
 7. The method of claim 1 wherein:the reducing is after the bending.
 8. The method of claim 1 wherein: thereducing is along an inner diameter face and an outer diameter face ofthe seed.
 9. The method of claim 1 wherein: the reducing is at aplurality of locations circumferentially spaced in the ultimate bentseed.
 10. The method of claim 9 wherein: the reducing creates a throughcut at each of the locations, optionally open to the first end of theseed.
 11. The method of claim 1 wherein: the molten material solidifiesin an annular form.
 12. (canceled)
 13. (canceled)
 14. (canceled) 15.(canceled)
 16. (canceled)
 17. (canceled)
 18. A casting methodcomprising: forming a seed, the seed having a first end and a secondend, the forming including bending a seed precursor; placing the seedsecond end in contact or spaced facing relation with a chill plate;contacting the first end with molten material; and cooling andsolidifying the molten material so that a crystalline structure of theseed propagates into the solidifying material, the forming furthercomprising at least one of: rendering a portion of the seed precursorproximate the first end less prone to stress from the bending than aportion of the seed precursor proximate the second end; relieving astress in the seed proximate the first end relative to a stress in theseed proximate the second end; and removing stressed material in theseed proximate the first end relative to stressed material in the seedproximate the second end.
 19. The method of claim 18 wherein: theforming includes said rendering; and said rendering comprises removingmaterial.
 20. The method of claim 18 wherein: the forming includes saidrelieving; and said relieving comprises removing material.
 21. Themethod of claim 18 wherein: during the casting, a melt back region ofthe seed is limited to at least one of the portion of the seed precursorproximate the first end; a portion of the seed proximate the first endand subject to said relieving; and a portion of the seed proximate thefirst end and subject to said removing material.
 22. The method of claim21 wherein: the at least one of the rendering, relieving, and removingcomprises said removing.
 23. The method of claim 1 wherein: the reducingthe thickness of the seed proximate the first end relative to thethickness of the seed proximate the second end comprises reducing thethickness of a portion proximate the first end relative to the thicknessof a portion proximate the second end; and during the casting, a meltback region of the seed is limited to the portion proximate the firstend.
 24. The method of claim 23 wherein: the reducing the thickness ofthe seed proximate the first end relative to the thickness of the seedproximate the second end comprises: removing at least 5% of theprecursor thickness from both an inner diameter surface and an outerdiameter surface.
 25. The method of claim 1 wherein: during the casting,a melt back region of the seed is limited to the region of reducedthickness of the seed proximate the first end.
 26. The method of claim25 wherein: the reducing is along an inner diameter face and an outerdiameter face of the seed.